According to reports, the gunman called a friend to say goodbye before the final shootout that resulted in his death. The FBI has interviewed that friend, sources say, though no details of the conversation have been made public.

The other call Mateen reportedly made was to a local news station News13, where he announced his allegiance to the Islamic State.

"There could be as many as three or four others (dead), and possibly up to 20 people that have been injured."

Authorities first heard reports of a man being shot while driving. Then, another report came in of a man being shot in the leg. After another shooting occurred in the Excel parking lot, the suspect was confirmed to have entered the building.

Walton added:

"This is just a horrible incident that's happened here… It's going to be a lot of sad people before this is all over."